The present invention relates to the field of medical devices with limited data storage capacity. More particularly, the present invention relates to implantable devices, such as cardiac pacing systems, that are capable of compressing and storing data, especially cardiac signal data such as episodes of intracardiac electro cardiogram (IEGM).
Medical devices with limited data storage capacity are well known in the art. Two common examples are hearing aids and pacemakers. In the latter case, some pacemakers, or other such implantable pulse generators (IPGs) include means for storing data related to cardiac events. These cardiac events may include, for example, episodes of spontaneous heart rate that are higher or lower than an acceptable or previously established rate. Stored data related to one or more cardiac events are useful in assessing the functioning of the IPG and in monitoring the progress of the patient.
Digital signal processing (DSP) has proved to be a useful tool in the environment of medical devices such as implantable pulse generators. Using DSP technology, an incoming sensed heart signal may be converted to a digital signal, e.g., an 8-bit signal. This conversion may occur at a predetermined sample rate. For example, episodes of IEGM may be processed using DSP. The IEGM is one type of signal in which heart contractions may be identified.
Typically an input signal from an IPG is amplified. The signal may then be converted to a digital signal (using, for example, analog to digital (A/D) converters). Then the signal may be digitally processed, generally by filtering the resulting digital data streams. The result from this process is generally a number of digital data streams. Each data stream is more or less a digitally processed representation of an IPG input signal. Based upon the information in these streams, DSP technology may be used to determine heart contractions. As stated above, a physician may use information about these contractions to assess and monitor the efficacy of IPG therapy.
Typically, data is collected continuously while the patient is using the IPG. However, a physician is only able to view the data when the patient and the IPG are available for evaluation, e.g. when the patient is in the physician""s office. Usually, the IPG is linked to an interrogation device with a display, which shows the data being collected at the time the patient is being examined.
However, the most interesting episodes of IEGM generally occur when the patient is proceeding about his normal business, away from the physician""s office. Thus, current IPGs (and other implantable therapeutic devices) have the capability to store data, such as an IEGM, for later viewing by the physician. At the time of viewing, the IPG may be linked to an interrogation device with a display that communicates the stored data. Because implantable devices are, of necessity, small enough for implantation in a human body, their available storage space is limited. Thus, the signal, such as a digital IEGM, needs to be compressed as much as possible without losing the sense of the original signal. Moreover, the type of information stored is also important in analysis of the efficacy of IPG therapy. For example, storage of an episode may begin when a preset condition, or xe2x80x9ctriggerxe2x80x9d, has been met, e.g. the IPG senses a particular heart rate indicative of a cardiac event such as an atrial fibrillation. Although data representing the cardiac event, or trigger, itself may be of interest to the physician, analysis of the data representing cardiac conditions prior to the event may be more useful in assessing the device""s performance and the disease""s progress. Thus, the ability to store data prior to the xe2x80x9ctriggerxe2x80x9d event is also desirable. The longer the stored episode, particularly prior to the xe2x80x9ctriggerxe2x80x9d, the more useful the data is to the physician. Again, efficient compression would allow longer episodes to be stored.
Thus, a need exists in the medical arts for compressing and storing data in an implantable medical device.
Several methods have been proposed in the prior art for improving compression and storage in an implantable medical device.
For example, U.S. Pat. No. 5,603,331 to Heemels et al., entitled xe2x80x9cData Logging System For Implantable Cardiac Devicexe2x80x9d discloses the compression of heart rate variability data via logarithmic data compression and the storing of the results as time-related histograms with a standard deviation.
U.S. Pat. No. 5,819,740 to Muhlenberg entitled xe2x80x9cSystem and Method for Compressing Digitalized Signals in Implantable and Battery-Powered Devicesxe2x80x9d discloses the compression of data using non-linear sampling. A time varying threshold is used and the signal of interest is compared to the threshold.
U.S. Pat. No. 5,836,982 to Muhlenberg et al., entitled xe2x80x9cSystem and Method of Data Compression and Non-Linear Sampling from Implantable and Battery-Powered Devicesxe2x80x9d discloses compressing a data block by storing the change, or delta, from one sample to another sample.
U.S. Pat. No. 5,312,446 to Holschbach et al., entitled xe2x80x9cCompressed Storage of Data in Cardiac Peacemakersxe2x80x9d discloses compression of data using an analog implementation of a turning point algorithm.
U.S. Pat. No. 5,623,935 to Faisandier entitled xe2x80x9cData Compression Methods and Apparatus for Use with Physiological Dataxe2x80x9d discloses compression of data by generating the first and second derivatives of an analog signal. The first and second derivatives of an analog signal are generated and one of three modes of encoding is selected. Either one of the derivative values is then encoded using one of the three modes based upon maximum compression.
U.S. Pat. No. 5,709,216 to Woodson entitled xe2x80x9cData Reduction of Sensed Values in an Implantable Medical Device Through the Use of a Variable Resolution Techniquexe2x80x9d discloses compression of data using variable resolution. The variable resolution is based upon pre-selected sub-ranges, i.e., smaller values or intervals have finer resolutions.
U.S. Pat. No. 5,215,098 to Steinhause et al., entitled xe2x80x9cData Compression of Cardiac Electrical Signals Using Scanning Correlation and Temporal Data Compressionxe2x80x9d discloses data compression by storing pre-recorded (i.e. learned) signal templates.
U.S. Pat. No. 5,217,021 to Steinhause et al., entitled xe2x80x9cDetection of Cardiac Arrhythmias Using Correlation of a Cardiac Electrical Signal and Temporal Data Compressionxe2x80x9d also discloses data compression using stored pre-recorded signal templates.
U.S. Pat. No. 5,836,889 to Wyborny et al., entitled xe2x80x9cMethod and Apparatus for Storing Signals in an Implantable Medical Devicexe2x80x9d discloses compression of data for storing a straight-line connection between the last stored value and new data. Data is stored when the first derivative exceeds a threshold.
U.S. Pat. No. 4,716,903 to Hanson et al., entitled xe2x80x9cStorage in a Pacemaker Memoryxe2x80x9d discloses data compression by storing the time to the next sample. The time is stored when the samples are near the baseline. An additional flag is added for turning points.
U.S. Pat. No. 5,263,486 to Jeffreys entitled xe2x80x9cApparatus and Method for Electrocardiogram Data Compressionxe2x80x9d discloses data compression by varying the sampling period dynamically. The variation is based upon signal rate of change value.
U.S. Pat. No. 4,920,489 to Hubelbank et al., entitled xe2x80x9cApparatus and Method for Solid State Storage of Episodic Signalsxe2x80x9d discloses compression of data by storing the derivative value, which is defined as data differing from the last stored value. The resolution is also changed based upon the magnitude of rate change.
U.S. Pat. No. 5,735,285 to Albert et al., entitled xe2x80x9cMethod and Hand-Held Apparatus for Demodulating and Viewing Frequency Modulated Biomedical Signalsxe2x80x9d discloses transmission of data using A-Law encoding and decoding.
U.S. Pat. No. 5,694,356 to Wong et al., entitled xe2x80x9cHigh Resolution Analog Storage EPROM and Flash EPROMxe2x80x9d discloses compression of a signal using A-Law or U-Law log arrhythmic relationships.
As discussed above, the most pertinent prior art patents are shown in the following table:
All the patents listed in Table 1 are hereby incorporated by reference herein in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, the Detailed Description of the Preferred Embodiments and the claims set forth below, many of the devices and methods disclosed in the patents of Table 1 may be modified advantageously by using the teachings of the present invention.
The present invention is therefore directed to providing a method and system for compressing and storing data in an implantable medical device, such as a cardiac pacing device. The system of the present invention may overcome at least some of the problems, disadvantages and limitations of the prior art described above, and provides a more efficient and accurate means of compressing and storing data, such as heart signal data, in an implantable medical device.
The present invention has certain objects. That is, various embodiments of the present invention provide solutions to one or more problems existing in the prior art respecting the pacing of cardiac tissue. Those problems include, without limitation: (a) limited data storage capacity of an implantable device; (b) limited data compression capabilities of an implantable device; (c) difficulty in distinguishing data which may indicate or precede a trigger event; (d) difficulty in determining which direction to encode or decode the data stream (e) variability in data values being compressed or otherwise processed by an implantable medical device; and (f) difficulty in identifying start of storage location, end of storage location and/or significant event location in a particular data stream being compressed.
In comparison to known techniques for storing data in an implantable device, various embodiments of the present invention may provide one or more of the following advantages: (a) increased data storage capacity in an implantable device; (b) efficient and accurate processing of data streams in an implantable device; (c) ability to compress and store of data streams that include markedly differing values in an implantable device; (d) ability to indicate a value corresponding to a significant event, a start of storage and/or an end of storage; and (e) the ability to store efficiently data and identify data gathered before and/or including a significant event.
Some of the embodiments of the present invention include one or more of the following features: (a) an implantable device with increased data storage capacity; (b) an implantable device capable of efficient compression and storage of data streams that include markedly differing values; (c) an implantable device capable of indicating the location of a significant event value, a start of storage value and/or an end of storage value within a data stream; (d) methods for compressing data streams that include markedly differing values; (e) methods of compressing data streams in which the location of a significant event value, a start of storage value and/or an end of storage value are indicated within the compressed data stream; and (f) methods of identifying data gathered before and/or including a significant event.
At least some embodiments of the present invention involve receiving a current data sample. A difference value is determined between the current data sample and a previous data sample. The previous data sample may also be received. The current data sample is compressed to at least one output symbol based on the difference value between the current sample and the previous data sample. The series of output symbols may comprise more than one character. The amplitude resolution of the current data sample may be reduced. A sequence of similar output symbols may be converted to an escape output symbol. The series of output symbols may be converted to a variable bit length code. An absolute reference value may be inserted after the current sample.